Traction drive power transmissions, which transmit power to a driven part through a traction drive mechanism, have recently attracted attention in the field of automobiles and industrial machinery. The traction drive mechanism is a power transmitting mechanism using a rolling friction. Unlike conventional drive mechanisms it does not use any gears, which results in reduced vibration and noise as well as a smooth speed change in high-speed rotation. An important goal in the automobile industry is improvement in the fuel consumption of automobiles. It has been suggested that if the traction drive is applied to the transmission of automobiles to convert the transmission to the continuous variable-speed transmission fuel consumption can be reduced by at least 20% compared to conventional transmission systems. This is due to the fact that the drive can always be in the optimum speed ratio. Recent studies have resulted in the development of materials having high fatigue resistance as well as in a theoretical analysis of traction mechanisms. As regards the traction fluid the correlation of traction coefficients is gradually being understood on a level of the molecular structure of the components. The term "traction coefficient" as used herein is defined as the ratio of the tractional force which is caused by slipping at the contact points between rotators which are in contact with each other in a power transmission of the rolling friction type to the normal load.
The traction fluid is comprised of a lubricating oil having a high traction coefficient. It has been confirmed in this connection that a traction fluid possessing a molecular structure having a naphthene ring exhibits high performance. "Santotrack.RTM." manufactured by the Monsanto Chemical Company is widely known as a commercially available traction fluid. Japanese patent publication No. 35763/1972 discloses di(cyclohexyl)alkane and dicyclohexane is traction fluids having a naphthene ring. This patent publication discloses that a fluid obtained by incorporating the above-mentioned alkane compound in perhydrogenated (.alpha.-methyl)styrene polymer, hydrindane compound or the like has a high traction coefficient. Further, Japanese patent laid-open No. 191797/1984 discloses a traction fluid containing an ester compound having a naphthene ring. It discloses that an ester obtained by the hydrogenation of the aromatic nucleus of dicyclohexyl cyclohexanedicarboxylate or dicyclohexyl phthalate is preferred as the traction fluid.
As mentioned above, in recent years there has been progress in the development of continuous variable-speed transmissions. The higher the traction coefficient of the lubricating fluid the larger the transmission force in the same device. This allows a reduction in size of the entire device with a concomitant reduction in polluting exhaust gases. Therefore, there is a demand for a fluid having a traction coefficient as high as possible. However, even the use of Santotrack.RTM., which is a traction fluid having the highest performance of all the currently commercially available fluids, in such a traction drive device provides unsatisfactory performance with respect to the traction coefficient, and is also expensive. The traction fluid which has been proposed in Japanese patent publication No. 35763/1971 contains Santotrack.RTM. or its analogue as a component and, therefore, is also unsatisfactory with respect to its performance and cost
The present inventors have made extensive and intensive studies with a view to developing a traction fluid which not only exhibits a high traction coefficient but is also inexpensive. As a result, the inventors have found that the use of a diester or its derivative having two cyclohexyl rings can economically provide a high-performance base oil fluid. The present invention has been made based on this finding.